Sizing agents are generally used for providing a water resistant paper which thereby is not subject to record-blotting. In conventional acid papermaking process, paper is treated with a rosin sizing agent, etc. together with aluminum sulfate as a fixer to obtain required sizing degree. Aluminum sulfate is also effective for avoiding the generation of pitch and slime. However, when aluminum sulfate is used, dehydration reaction occurs due to the existence of SO.sub.4.sup.2-. Thus, paper deterioration problems such as yellowing and decrease in strength of paper are encountered. 1-2% of Aluminum sulfate based on weight of pulp is usually added in the acid papermaking process, when the pH of pulp slurry becomes 4-5. As shown in FIG. 1, since the effect of rosin sizing agent declines rapidly at a pH of 6 or more than 6 when an attempt is made to neutralize pulp slurry for the purpose of avoiding deterioration, paper having excellent anti-deterioration properties can not be manufactured.
On the other hand, there is a so-called neutral papermaking process providing a paper having excellent anti-deterioration properties. Alkyl ketene dimer as a sizing agent, alkenyl succinic anhydride, etc. as a neutral sizing agent, cationic high polymer as a yield activator in addition to calcium carbonate as a filler are used in such a process. In neutral papermaking technology, the pH of a paper extract is approximately 7.9-9.0, but, the pH of a pulp slurry for papermaking and the pH of a paper extract are similarly alkaline, and thus the whole paper layer structure exhibits the same pH level. As compared with conventional acid papermaking methods, there have been many problems with respect to workability due to the generation of pitch and slime or dirt resulting from a neutral sizing agent, etc. and higher costs are incurred.